If there is the possibility to make a mistake, somebody will certainly make it. You expect all the humans involved to be competent. But relying on that competence is a mistake. The emotional stress of dealing with such enormous responsibilities, the often long work hours and the long list of procedures will make any competent professional to inadvertently slip up at some point.
In case of electrical connectors, the connectors are often grouped together in such a way as to avoid making wrong connections. Connectors with different sizes, keying, gender, etc are chosen to make this happen. This precaution is taken at design time. JPL is extremely experienced in these matters. There is probably something else left unsaid, that led to this mistake being possible.
Meanwhile, motor controllers using H-bridge is something that's never boring. I once saw a motor control fail so spectacularly that we were scratching our heads for days afterwards. As always, a failure is never due to a single cause (due to careful design and redundancies). It's a chain of seemingly innocuous events with a disastrous final outcome. But the chain was so mind-bending that we had to write it down just to remember how it happened. Recently, I was watching the Chernobyl nuclear disaster and I got reminded of this failure. Our failure was nowhere near as disastrous - but the initial mistakes, the control system instability, the human intervention and the ultimate failure propagation were very similar in nature. Needless to say, it sent us back to the drawing board for a complete redesign. The robustness of the final design taught me the same lesson - failures are something you take advantage of.